An object to be coated using electrodeposition, such as the body of an automobile, often includes steel plate mating portions, such as a bag-shaped portion in a door included in the body of an automobile.
The steel plate mating portions are gaps between steel plates, the entrances of which are narrowed due to steel plates being brought into contact with each other when the object to be coated is processed, so that communication with the outside is limited.
When electrodeposition is performed, during an electrodeposition process in which the object to be coated is immersed in a coating material solution for electrodeposition, and a washing process in which the coated object that has undergone the electrodeposition process is washed using washing water, the coating material solution and washing water used in these processes enter into the steel plate mating portions.
The coating material solution and washing water that have entered into the steel plate mating portions are likely to remain in the steel plate mating portions in the form of a coating material-containing aqueous solution in which the coating material solution and washing water are mixed with each other, even after the washing process for the coated object is completed.
Therefore, during the drying process in which the coated object is heated so that the coating is hardened and dried, a coating material-containing aqueous solution remaining in the steel plate mating portions rapidly boils due to rapid heating, and flows out of the steel plate mating portions, and a coating material-containing aqueous solution that has flowed out adheres to a neighboring portion of the coating of the coated object.
Then, a coating material component contained in the coating material-containing aqueous solution adhering to the coating is dried during the drying process together with the coating, in the state of adhering to the coating.
Conventionally, there is a problem in which the final quality of the coating of a coated object is significantly degraded due to the above-described phenomenon (degradation in the final quality of a coating due to so-called “secondary sagging”).
To address such a problem, Patent Document 1 below, for example, proposes an electrodeposition method in which, after a coated object that has undergone the electrodeposition process is washed using washing water, the coated object is subjected to a pre-heating process during which the coated object is heated in a pre-drying furnace, and to a hot water spraying process subsequent to the pre-heating process, in which the steel plate mating portions of the coated object are sprayed with hot water or hot water mist.
This proposed method utilizes the fact that, if the temperature of the coating material-containing aqueous solution is increased, the surface tension and viscosity of the coating material-containing aqueous solution decrease, and consequently the coating material-containing aqueous solution is more likely to flow out of the steel plate mating portions.
That is, according to the proposed method, the temperature of a coating material-containing aqueous solution remaining in the steel plate mating portions rises during the pre-heating process in which the coated object is heated in a pre-drying furnace so that the surface tension and viscosity of the coating material-containing aqueous solution decrease.
Then, during the hot water spraying process that is subsequent to the pre-heating process, the steel plate mating portions are sprayed with hot water or hot water mist to keep the temperature of a coating material-containing aqueous solution remaining in the steel plate mating portions at a high temperature, to maintain a state in which the surface tension and viscosity of the coating material-containing aqueous solution are low.
Thus, a coating material-containing aqueous solution remaining in the steel plate mating portions is removed before the coated object is subjected to a drying process, by causing the coating material-containing aqueous solution to drip from the steel plate mating portions.